This invention relates generally to pulse compression systems, and more particularly to means for reducing the range sidelobes of the compressed pulse produced in such sysems.
Pulse-compression techniques are used in radar systems in order to increase the energy of each radar pulse radiated without losing time resolution and without encountering electrical breakdown in the radiating system caused by high peak power. The radar transmits a long coded pulse with maximum allowable peak power and a bandwidth equal to the reciprocal of the desired time resolution. The received echo is processed through a matched filter which produces a pulse of length equal to the reciprocal of the transmitted signal bandwidth which is much shorter than the time length of the transmitted pulse. Pulse compression increases signal energy and range resolution and reduces clutter.
One method of implementing pulse compression in a radar system is to transmit pulses which consist of a carrier modulated according to a certain code pattern. In the radar receiver, a matched filter is provided by means of which a correlation between the incoming radar pulse and the known pattern is carried out. As a result, there is obtained a signal with a well-defined autocorrelation peak surrounded by a number of range-time sidelobes. An example of such a known method to transmit and detect phase-coded radar pulses is described in the U.S. Pat. No. 4,156,876.
The sidelobes appearing in the filtered radar pulse are not desirable for the following reason. When the radar is detecting two different targets A and B, situated at a certain distance from each other, the echo from the target A may interfere with the echo from the target B within a certain mutual distance x between the radar targets. This distance is determined by the pulse length T transmitted by the radar and the velocity of light C as x=CT/2. If the distance is so short that the echo pulse from B appears during the time interval when the echo pulse from the target A appears, interference is obtained. If the target A gives rise to a strong echo while the target B gives rise to a weak echo, the autocorrelation peak in the pulse reflected from B can be completely hidden by the sidelobes in the pulse reflected from A. Thus, there is a risk that the target B can not be discovered by the receiver. Therefore, it is important that the sidelobes of the reflected pulses be minimized in order to avoid the possibility that the weak target echos will be hidden by the sidelobes from an adjacent stronger target echo.
Furthermore, the presence of the sidelobes are disadvantageous as they contribute to the noise level when detecting a target in clutter, that is, unwanted radar echos from ground, sea, rain, etc.
Old methods and apparatus produce autocorrelation function peak to peak-sidelobe ratios less than or equal to 10 times the time-bandwidth product of the signal for large time-bandwidth products unless amplitude weighting is employed. Such weighting, however, reduces the output signal-to-noise ratio and complicates the compressor. A typical example is a Frank Code compressor as discussed in Cook and Bernfeld, "Radar Signal", 1967, Academic Press. These peak to peak-sidelobe ratios are not large enough to meet the needs of radars that must range resolve targets whose magnitudes may differ by many orders of magnitude and which must obtain as high a signal-to-noise ratio as possible.